The formal verification of FBD program is required in nuclear engineering domain as traditional relay-based analog systems are being replaced with digital PLC based software. This paper proposes a way to formally verify the FBD program. For this purpose, Verilog model is automatically translated from the FBD program, then Cadence SMV performs model checking. We demonstrated the effectiveness of the suggested approach by conducting a case study of the nuclear reactor protection system, which is currently being developed in Korea.